Transit and Congestion, an Indirect Connection

Yesterday, Freakonomics linked to a new piece of research [PDF] on congestion that I'd been musing over for a few days. Let me quote the abstract here (paragraph break and emphasis mine):

We investigate the relationship between interstate highways and highway vehicle kilometers traveled (vkt) in us cities. We find that vkt increases proportionately to highways and identify three important sources for this extra vkt: an increase in driving by current residents; an increase in transportation intensive production activity; and an inflow of new residents.

The provision of public transportation has no impact on vkt. We also estimate the aggregate city level demand for vkt and find it to be very elastic. We conclude that an increased provision of roads or public transit is unlikely to relieve congestion and that the current provision of roads exceeds the optimum given the absence of congestion pricing.

The first inclination of most urbanists, when confronted with something like this, is probably to bristle and conclude that the authors are nuts. Transit can't relieve congestion? What would happen to New York or Washington if transit systems were shut down for a day? There would be chaos!

Photo by derang0.

There would indeed be chaos. But that doesn't mean that the authors are wrong, and it's important to understand why.

When a transit system is built, two things happen to area roadways. Some subset of drivers will find that commuting by transit is faster, or cheaper, or more convenient (or all three) than driving and will switch to transit. If that were where things ended, then transit construction would indeed reduce congestion.

But there is a knock-on effect. When drivers switch to transit, roads become less congested, and driving therefore becomes more attractive. Drivers who were previously commuting at off-peak times to avoid congestion will switch to peak times, and drivers who were otherwise adapting to congestion by working at home some days or taking longer, alternative routes will adjust their behavior as well.

The end result will be ... a return to road congestion. This may not happen immediately. When new capacity of any sort -- roads or rails -- is built, there may be a time period during which traffic flows more freely, but ultimately settlement and transportation patterns will adjust until roads are again congested.

This happens because roads are under-priced (and often free). If drivers don't have to pay to use scarce road space, and don't have to pay to cover the cost of congestion their driving imposes on others, then drivers will use the road until it is congested. Because the government isn't using price to ration demand (as is done with most consumer goods), demand will rise until the cost of lost time rations demand, and pushes drivers to take other routes or modes.

I don't think there's any point in denying this. Instead, there are key points that urbanists should take to heart.

One is that roads are overbuilt. Years of trying to fight congestion, ineffectually, by building new capacity have generated a road network that is far too large.

Another is that road congestion can only be addressed through imposed rationing, and the most efficient way of doing this rationing is through variable tolling. This, of course, should raise a lot of new and desperately needed revenue.

The third point is that just because transit can't permanently reduce congestion doesn't mean that there aren't good reasons to build transit. Indeed, the authors of the above study conclude that it will often make sense to do so.

What kinds of reasons? Well, just because the road network is overbuilt doesn't mean that all transportation demand is adequately met. In many areas -- particularly in center cities and job concentrations -- new capacity is needed, and transit offers an effective way to move a lot of people through dense areas.

Another reason is that in the busiest areas, congestion tolls are likely to be fairly high, and transit can offer an affordable alternative to driving to low- and middle-income travelers.

And finally, transit can help address one of the key contributors to transportation demand -- poor land use. Transit networks offer a framework around which denser, mixed-use neighborhoods can be built, allowing more people to walk or bike to work or reduce their travel needs. Infrastructure is expensive to build and maintain, and ideally we'd like to grow in a more intensive way that reduces that needed infrastructure for a given amount of land.

Urbanists ought to trumpet these results. Drivers are more likely to accept congestion tolls if it is made clear to them that such tolls are the only way to reduce road congestion. And tolling will create both the demand and the revenue for new transit capacity. The relationship between capacity and congestion is one we'd all do well to understand.

Note that traffic congestion is only one of two congestions created by cars – the other is parking congestion. And if traffic congestion is at the maximum level that people are “willing to pay”, taking people off the road will indeed generate “more” traffic to bring congestion back to the level people are “willing to pay”.

However, to the extent that some of that is people who wanted to drive during peak hours switch and drive off-peak to avoid the congestion, and they switch back – that is not substantially more peak parking demand.

So while the traffic congestion may not be reduced, the parking congestion will be, driven by the share of driving trips are being shifted off-peak as opposed to being eliminated by the reaction to present traffic congestion.

http://cityblock.wordpress.com Alex B.

This is why I always cringe a little when transit advocates talk about a new line “reducing” congestion. It often doesn’t reduce anything, but it certainly does help the area deal with congestion.

I can’t think of a better way to frame the issue, however – No other short sound byte has the same simplicity – which is of course the core issue. “reducing” congestion is wrong because it’s a simple solution to a complex issue.

Charley

That’s an extremely interesting analysis. I have to admit I always would just think that transit would decrease congestion.

This means that there is an equilibrium amount of absolute congestion (I say absolute because I’m sure most of us would think that the equilibrium would be in congestion relative to other options available). Whenever it is lower, you have marginal people changing their behaviors to bring it back to its former level. My first thought was that the “knock-on” effect exists but there should still be a net decrease in congestion. However, this group of people who are marginally becoming drivers are probably LESS likely to opt for transit. Assuming some basic transit existed and it simply got better, they opted to stay home or switch hours rather than take transit. They have different preferences that would lean them in favor of driving, which explains the knock-on effect.

Velocentric

The corollary to this fine article is how installation or improvements of roads parallel to transit facilities almost guarantees their fiscal failure.

Transit policy work should include removal of subsidies given to autocentric facilities over other modes (in the form of free, overbuilt highways and destination parking). Then market forces can help balance transit mode choices and location density.

Graham

But transit does reduce transit dramatically…for those who take transit. And that is the point, isn’t it? Building transit for the benefit of car drivers is a crazy proposition.